How to change the text size of labels of additional ticks.

 At the bottom of the Format Axis dialog, you can enter information for additional ticks. At any coordinate, you can write any text.

Bug in Prism 5.0 Windows (up to 5.03): If you select the axis itself (or the axis segment, if the axis is discontinuous), the toolbar buttons to increase or decrease the text size (or to make the text bold or underline) don't work. Those buttons will only change the font of the regular ticks, but not the additional ticks. In contrast, with 5.0c (Mac), those buttons work to change both regular and additional ticks.

Workaround: To change the point size of these labels, click right on the number or text on the graph, and change its size or appearance. The trick is to click and select the number/label itself, rather than click to select the entire axis (or axis segment).